访问JavaScript对象的构造函数时失败

灰太狼 2022-01-05 10:09 171阅读 0赞

在JavaScript定义的对象中，不管是内部对象，还是用户自定的对象。如果该对象是从模态窗口(Modal Dialog)中创建并返回到主窗口中的，我们将无法在主窗口中取到该对象的构造函数(constructor)。  
  
执行如下两个示例：  
1、Main.htm

![None.gif][] < html >   
![None.gif][] < head >   
![None.gif][] < title > Main Page </ title >   
![None.gif][] < meta  name ="author"  content ="birdshome@博客园"  />   
![None.gif][] </ head >   
![None.gif][] < body >   
![None.gif][] < button  onclick ="GetValueFromDialog()" >   
![None.gif][] click </ button >   
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] < script > ![dot.gif][None.gif]   
![InBlock.gif][None.gif]var m\_Array = \[\];  
![InBlock.gif][None.gif]function GetValueFromDialog()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]var array = window.showModalDialog('dialog.htm');  
![InBlock.gif][None.gif] alert(array.constructor);  
![InBlock.gif][None.gif]// alert(new m\_Array.constructor);  
![InBlock.gif][None.gif]// alert(new array.constructor);  
![ExpandedSubBlockEnd.gif][None.gif] \}  
![ExpandedBlockEnd.gif][None.gif] </ script >   
![None.gif][] </ body >   
![None.gif][] </ html >   
![None.gif][]

2、Dialog.htm

![None.gif][] < html >   
![None.gif][] < head >   
![None.gif][] < title > Modal Dialog </ title >   
![None.gif][] < meta  name ="author"  content ="birdshome@博客园"  />   
![None.gif][] </ head >   
![None.gif][] < body >   
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] < script > ![dot.gif][None.gif]   
![InBlock.gif][None.gif]function ReturnValue()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]![dot.gif][None.gif]\{  
![InBlock.gif][None.gif] window.returnValue = \['modal dialog'\];  
![InBlock.gif][None.gif]// window.returnValue = new function foo()\{\};   
![InBlock.gif][None.gif] window.close();  
![ExpandedSubBlockEnd.gif][None.gif] \}  
![ExpandedBlockEnd.gif][None.gif] </ script >   
![None.gif][] < button  onclick ="ReturnValue()" > close </ button >   
![None.gif][] </ body >   
![None.gif][] </ html >   
![None.gif][]

关闭弹出窗口dialog.htm，执行alert(array.constructor);将会引发脚本运行时异常：

![None.gif][] A Runtime Error has occurred.  
![None.gif][] Do you wish to Debug ?   
![None.gif][]  
![None.gif][] Line:  12   
![None.gif][] Error: Unexpected call to method or property access.

// Unable to evaluate the expression. Catastrophic failure  
  
不过在这里JavaScript的内部对象和用户自定义对象还有一点小区别，如果是JS内部对象，我们访问对象的构造函数就立即出错。alert(array.constructor)就异常了。而如果是用户指定一对象，我们可以执行alert(array.constructor)得到一个显示"\[object\]"的MsgBox，但是这时的contrutor仍然不支持任何的操作和执行任何方法，比如new、.toString()等，一旦执行就出和上面一样的异常"Unable to evaluate the expression. Catastrophic failure"。  
  
这个缺陷似乎对我们的影响不是很大，不过对于一些依赖于对象的constructor来实现的功能就郁闷了，比如 [获取用户自定义对象的名称][Link 1]、 [JavaScript面向对象编程之Singleton类][JavaScript_Singleton]、以及我上次做的一个 [对象Clone方法][Clone]等，在这中情况下就都歇菜了 ![hitwall.gif][None.gif]。

转载于:https://www.cnblogs.com/birdshome/archive/2005/04/01/122075.html

[None.gif]: 
[Link 1]: http://www.cnblogs.com/birdshome/archive/2005/02/06/99564.html
[JavaScript_Singleton]: http://www.cnblogs.com/birdshome/archive/2005/02/27/109617.html
[Clone]: http://www.cnblogs.com/birdshome/archive/2005/03/20/122246.html